Power consumption in motion-capture systems

ABSTRACT

The technology disclosed relates to reducing the overall power consumption of motion-capture system without compromising the quality of motion capture and tracking In general, this is accomplished by operating the motion-detecting cameras and associated image-processing hardware in a low-power mode unless and until a moving object is detected. Once an object of interest has been detected in the field of view of the cameras, the motion-capture system is “woken up,” i.e., switched into a high-power mode, in which it acquires and processes images at a frame rate sufficient for accurate motion tracking.

FIELD OF THE TECHNOLOGY DISCLOSED PRIORITY DATA

This application claims the benefit of U.S. Provisional PatentApplication No. 61/749,638, filed Jan. 7, 2013, entitled “OPTIMIZINGPOWER CONSUMPTION IN MOTION-CAPTURE SYSTEMS”, (Atty Docket No. ULTI1028-2). The provisional application is hereby incorporated by referencefor all purposes.

The technology disclosed relates, in general, to image acquisition andanalysis, and in particular implementations to capturing motions ofobjects in three-dimensional space.

BACKGROUND

Power requirements, however, can pose a practical limit to the range ofapplications of motion-capture systems, as excessive power consumptioncan render their employment impractical or economically infeasible. Itwould therefore be desirable to reduce power consumption ofmotion-capture systems, preferably in a manner that does not affectmotion-tracking performance.

SUMMARY

The technology disclosed provides systems and methods for reducing theoverall power consumption of motion-capture system without compromisingthe quality of motion capture and tracking. In one implementation, thisis accomplished by operating the motion-detecting cameras and associatedimage-processing hardware in a low-power mode (e.g., at a low frame rateor in a standby or sleep mode) unless and until a moving object isdetected. Once an object of interest has been detected in the field ofview of the cameras, the motion-capture system is “woken up,” i.e.,switched into a high-power mode, in which it acquires and processesimages at a frame rate sufficient for accurate motion tracking. Forexample, in a computer system that interprets a user's hand gestures asinput, the requisite motion-capture system associated with the computerterminal may be idle, i.e., run in a low-power mode, as long aseverything in the camera's field of view is static, and start capturingimages at a high rate only when a person has entered the field of view.This way, high power consumption is limited to time periods when itresults in a corresponding benefit, i.e., when the system is activelyused for motion capture.

The motion detection that triggers a “wake-up” of the motion-capturesystem can be accomplished in several ways. In some implementations,images captured by the camera(s) at a very low frame rate are analyzedfor the presence or movement of objects of interest. In otherimplementations, the system includes additional light sensors, e.g.,located near the camera(s), that monitor the environment for a change inbrightness indicative of the presence of an object. For example, in awell-lit room, a person walking into the field of view near thecamera(s) may cause a sudden, detectable decrease in brightness. In amodified implementation applicable to motion-capture systems thatilluminate the object of interest for contrast-enhancement, the lightsource(s) used for that purpose in motion-tracking mode are blinked, andreflections from the environment captured; in this case, a change inreflectivity may be used as an indicator that an object of interest hasentered the field of view.

In some implementations, the motion-capture system is capable ofoperation in intermediate modes with different rates of image capture.For example, the system can “throttle” the rate of image capture basedon the speed of the detected motion, or the rate may be reset in realtime by the user, in order to maximally conserve power.

Other aspects and advantages of the present technology can be seen onreview of the drawings, the detailed description and the claims, whichfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, with an emphasis instead generally being placedupon illustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 illustrates a system for capturing image data according to animplementation of the technology disclosed.

FIG. 2 shows a flowchart of one implementation of operating amotion-capture system with reduced power consumption.

FIG. 3 is a block diagram of an example computer system for operating amotion-capture system with reduced power consumption.

DESCRIPTION

The term “motion capture” refers generally to processes that capturemovement of a subject in three-dimensional (3D) space and translate thatmovement into, for example, a digital model or other representation.Motion capture is typically used with complex subjects that havemultiple separately articulating members whose spatial relationshipschange as the subject moves. For instance, if the subject is a walkingperson, not only does the whole body move across space, but thepositions of arms and legs relative to the person's core or trunk areconstantly shifting. Motion-capture systems are typically designed tomodel this articulation.

Motion-capture systems may utilize one or more cameras to capturesequential images of an object in motion, and computers to analyze theimages to create a reconstruction of an object's shape, position, andorientation as a function of time. For 3D motion capture, at least twocameras are typically used. Image-based motion-capture systems rely onthe ability to distinguish an object of interest from a background. Thisis often achieved using image-analysis algorithms that detect edges,typically by comparing pixels to detect abrupt changes in color and/orbrightness. Conventional systems, however, suffer performancedegradation under many common circumstances, e.g., low contrast betweenthe object of interest and the background and/or patterns in thebackground that may falsely register as object edges.

Object recognition can be improved by enhancing contrast between theobject and background surfaces visible in an image, for example, bymeans of controlled lighting directed at the object. For instance, in amotion-capture system where an object of interest, such as a person'shand, is significantly closer to the camera than any backgroundsurfaces, the falloff of light intensity with distance (1/r2 for pointlike light sources) can be exploited by positioning a light source (ormultiple light sources) near the camera(s) or other image-capturedevice(s) and shining that light onto the object. Source light reflectedby the nearby object of interest can be expected to be much brighterthan light reflected from more distant background surfaces, and the moredistant the background (relative to the object), the more pronounced theeffect will be. Accordingly, a threshold cut off on pixel brightness inthe captured images can be used to distinguish “object” pixels from“background” pixels. While broadband ambient light sources can beemployed, various implementations use light having a confined wavelengthrange and a camera matched to detect such light; for example, aninfrared source light can be used with one or more cameras sensitive toinfrared frequencies.

In order to accurately track motion in real or near-real time, thecamera(s) of motion-capture systems typically operate at a frame rate ofat least 15 image frames per second. Image acquisition at such highrates entails significant power requirements; in general, there is atrade-off between the frame-rate-dependent accuracy and responsivenessof motion-capture systems on the one hand and power consumption on theother hand. FIG. 1 illustrates a system 100 for capturing image dataaccording to an implementation of the technology disclosed. The system100 includes a pair of cameras 102, 104 coupled to a control andimage-processing system 106. (The system 106 is hereinafter variablyreferred to as the “control and image-processing system,” the “controlsystem,” or the “image-processing system,” depending on whichfunctionality of the system is being discussed.) The cameras 102, 104can be any type of camera (e.g., CCD or CMOS cameras), including camerassensitive across the visible spectrum or, more typically, with enhancedsensitivity to a confined wavelength band (e.g., the infrared (IR) orultraviolet bands); more generally, the term “camera” herein refers toany device (or combination of devices) capable of capturing an image ofan object and representing that image in the form of digital data. Forexample, line sensors or line cameras rather than conventional devicesthat capture a two-dimensional (2D) image can be employed. The term“light” is used generally to connote any electromagnetic radiation,which may or may not be within the visible spectrum, and may bebroadband (e.g., white light) or narrowband (e.g., a single wavelengthor narrow band of wavelengths). The cameras 102, 104 are preferablycapable of capturing video images, i.e., successive image frames at aconstant rate of, typically, at least 15 frames per second (although noparticular frame rate is required). The capabilities of the cameras 102,104 are not critical to the technology disclosed, and the cameras canvary as to frame rate, image resolution (e.g., pixels per image), coloror intensity resolution (e.g., number of bits of intensity data perpixel), focal length of lenses, depth of field, etc. In general, for aparticular application, any cameras capable of focusing on objectswithin a spatial volume of interest can be used. For instance, tocapture motion of the hand of an otherwise stationary person, the volumeof interest might be defined as a cube approximately one meter inlength.

System 100 also includes a pair of light sources 108, 110, which can bedisposed, e.g., to either side of cameras 102, 104, and controlled bycontrol and image-processing system 106. Light sources 108, 110 can beinfrared light sources of generally conventional design, e.g., infraredlight-emitting diodes (LEDs), and cameras 102, 104 can be sensitive toinfrared light. Filters 120, 122 can be placed in front of cameras 102,104 to filter out visible light so that only infrared light isregistered in the images captured by cameras 102, 104. In someimplementations where the object of interest is a person's hand or body,use of infrared light can allow the motion-capture system to operateunder a broad range of lighting conditions and can avoid variousinconveniences or distractions that may be associated with directingvisible light into the region where the person is moving. However, aparticular wavelength or region of the electromagnetic spectrum isgenerally not required. Instead of LEDs, lasers or other light sourcescan be used. For laser setups, additional optics (e.g., a lens ordiffuser) may be employed to widen the laser beam (and make its field ofview similar to that of the cameras). Useful arrangements can alsoinclude short- and wide-angle illuminators for different ranges. Lightsources are typically diffuse rather than specular point sources; forexample, packaged LEDs with light-spreading encapsulation are suitable.

In operation, cameras 102, 104 are oriented toward a region of interest112 in which an object of interest 114 (in this example, a hand) and oneor more background objects 116 can be present. Light sources 108, 110are arranged to illuminate region 112. In some implementations, one ormore of the light sources 108, 110 and one or more of the cameras 102,104 are disposed below the motion to be detected, e.g., in the case ofhand motion, on a table or other surface beneath the spatial regionwhere hand motion occurs. This is an optimal location because the amountof information recorded about the hand is proportional to the number ofpixels it occupies in the camera images, and the hand will occupy morepixels when the camera's angle with respect to the hand's “pointingdirection” is as close to perpendicular as possible. Further, if thecameras 102, 104 are looking up, there is little likelihood of confusionwith background objects (clutter on the user's desk, for example) andother people within the cameras' field of view.

Control and image-processing system 106, which can be, e.g., a computersystem, can control the operation of light sources 108, 110 and cameras102, 104 to capture images of region 112. Based on the captured images,the image-processing system 106 determines the position and/or motion ofobject 114. For example, as a step in determining the position of object114, image-analysis system 106 can determine which pixels of variousimages captured by cameras 102, 104 contain portions of object 114. Insome implementations, any pixel in an image can be classified as an“object” pixel or a “background” pixel depending on whether that pixelcontains a portion of object 114 or not. With the use of light sources108, 110, classification of pixels as object or background pixels can bebased on the brightness of the pixel. For example, the distance (rO)between an object of interest 114 and cameras 102, 104 is expected to besmaller than the distance (rB) between background object(s) 116 andcameras 102, 104. Because the intensity of light from sources 108, 110decreases as 1/r2, object 114 will be more brightly lit than background116, and pixels containing portions of object 114 (i.e., object pixels)will be correspondingly brighter than pixels containing portions ofbackground 116 (i.e., background pixels). For example, if rB/rO=2, thenobject pixels will be approximately four times brighter than backgroundpixels, assuming object 114 and background 116 are similarly reflectiveof the light from sources 108, 110, and further assuming that theoverall illumination of region 112 (at least within the frequency bandcaptured by cameras 102, 104) is dominated by light sources 108, 110.These assumptions generally hold for suitable choices of cameras 102,104, light sources 108, 110, filters 120, 122, and objects commonlyencountered. For example, light sources 108, 110 can be infrared LEDscapable of strongly emitting radiation in a narrow frequency band, andfilters 120, 122 can be matched to the frequency band of light sources108, 110. Thus, although a human hand or body, or a heat source or otherobject in the background, may emit some infrared radiation, the responseof cameras 102, 104 can still be dominated by light originating fromsources 108, 110 and reflected by object 114 and/or background 116.

In this arrangement, image-analysis system 106 can quickly andaccurately distinguish object pixels from background pixels by applyinga brightness threshold to each pixel. For example, pixel brightness in aCMOS sensor or similar device can be measured on a scale from 0.0 (dark)to 1.0 (fully saturated), with some number of gradations in betweendepending on the sensor design. The brightness encoded by the camerapixels scales standardly (linearly) with the luminance of the object,typically due to the deposited charge or diode voltages. In someimplementations, light sources 108, 110 are bright enough that reflectedlight from an object at distance rO produces a brightness level of 1.0while an object at distance rB=2rO produces a brightness level of 0.25.Object pixels can thus be readily distinguished from background pixelsbased on brightness. Further, edges of the object can also be readilydetected based on differences in brightness between adjacent pixels,allowing the position of the object within each image to be determined.Correlating object positions between images from cameras 102, 104 allowsimage-analysis system 106 to determine the location in 3D space ofobject 114, and analyzing sequences of images allows image-analysissystem 106 to reconstruct 3D motion of object 114 using conventionalmotion algorithms.

It will be appreciated that system 100 is illustrative and thatvariations and modifications are possible. For example, light sources108, 110 are shown as being disposed to either side of cameras 102, 104.This can facilitate illuminating the edges of object 114 as seen fromthe perspectives of both cameras; however, a particular arrangement ofcameras and lights is not required. As long as the object issignificantly closer to the cameras than the background, enhancedcontrast as described herein can be achieved.

In accordance with various implementations of the technology disclosed,the cameras 102, 104 (and typically also the associated image-analysisfunctionality of control and image-processing system 106) are operatedin a low-power mode until an object of interest 114 is detected in theregion of interest 112. For purposes of detecting the entrance of anobject of interest 114 into this region, the system 100 further includesone or more light sensors 118 that monitor the brightness in the regionof interest 112 and detect any change in brightness. For example, asingle light sensor including, e.g., a photodiode that provides anoutput voltage indicative of (and over a large range proportional to) ameasured light intensity may be disposed between the two cameras 102,104 and oriented toward the region of interest. The one or more sensors118 continuously measure one or more environmental illuminationparameters such as the brightness of light received from theenvironment. Under static conditions—which implies the absence of anymotion in the region of interest 112—the brightness will be constant. Ifan object enters the region of interest 112, however, the brightness mayabruptly change. For example, a person walking in front of the sensor(s)118 may block light coming from an opposing end of the room, resultingin a sudden decrease in brightness. In other situations, the person mayreflect light from a light source in the room onto the sensor, resultingin a sudden increase in measured brightness.

The aperture of the sensor(s) 118 may be sized such that its (or theircollective) field of view overlaps with that of the cameras 102, 104. Insome implementations, the field of view of the sensor(s) 118 issubstantially co-existent with that of the cameras 102, 104 such thatsubstantially all objects entering the camera field of view aredetected. In other implementations, the sensor field of view encompassesand exceeds that of the cameras. This enables the sensor(s) 118 toprovide an early warning if an object of interest approaches the camerafield of view. In yet other implementations, the sensor(s) capture(s)light from only a portion of the camera field of view, such as a smallerarea of interest located in the center of the camera field of view.

The control and image-processing system 106 monitors the output of thesensor(s) 118, and if the measured brightness changes by a set amount(e.g., by 10% or a certain number of candela), it recognizes thepresence of an object of interest in the region of interest 112. Thethreshold change may be set based on the geometric configuration of theregion of interest and the motion-capture system, the general lightingconditions in the area, the sensor noise level, and the expected size,proximity, and reflectivity of the object of interest so as to minimizeboth false positives and false negatives. In some implementations,suitable settings are determined empirically, e.g., by having a personrepeatedly walk into and out of the region of interest 112 and trackingthe sensor output to establish a minimum change in brightness associatedwith the person's entrance into and exit from the region of interest112. Of course, theoretical and empirical threshold-setting methods mayalso be used in conjunction. For example, a range of thresholds may bedetermined based on theoretical considerations (e.g., by physicalmodeling, which may include ray tracing, noise estimation, etc.), andthe threshold thereafter fine-tuned within that range based onexperimental observations.

In implementations where the area of interest 112 is illuminated, thesensor(s) 118 will generally, in the absence of an object in this area,only measure scattered light amounting to a small fraction of theillumination light. Once an object enters the illuminated area, however,this object may reflect substantial portions of the light toward thesensor(s) 118, causing an increase in the measured brightness. In someimplementations, the sensor(s) 118 is (or are) used in conjunction withthe light sources 106, 108 to deliberately measure changes in one ormore environmental illumination parameters such as the reflectivity ofthe environment within the wavelength range of the light sources. Thelight sources may blink, and a brightness differential be measuredbetween dark and light periods of the blinking cycle. If no object ispresent in the illuminated region, this yields a baseline reflectivityof the environment. Once an object is in the area of interest 112, thebrightness differential will increase substantially, indicatingincreased reflectivity. (Typically, the signal measured during darkperiods of the blinking cycle, if any, will be largely unaffected,whereas the reflection signal measured during the light period willexperience a significant boost.) Accordingly, the control system 106monitoring the output of the sensor(s) 118 may detect an object in theregion of interest 112 based on a change in one or more environmentalillumination parameters such as environmental reflectivity that exceedsa predetermined threshold (e.g., by 10% or some other relative orabsolute amount). As with changes in brightness, the threshold changemay be set theoretically based on the configuration of the image-capturesystem and the monitored space as well as the expected objects ofinterest, and/or experimentally based on observed changes inreflectivity.

FIG. 2 shows a flowchart 200 of one implementation of operating amotion-capture system with reduced power consumption. In variousimplementations, changes in brightness or reflectivity as detected basedon the sensor measurements described above are used to control theoperation of the system 100 so as to minimize power consumption whileassuring high-quality motion capture; FIG. 2 illustrates a suitablecontrol method 200. Initially, the control system 106 operates thecameras in a low-power mode (step 202), such as a standby or sleep modewhere motion capture does not take place at all or a slowimage-acquisition mode (e.g., with image-acquisition rates of fiveframes per second or less). This does not only reduce power consumptionby the cameras, but typically also decreases the power consumption ofthe control and image-processing system 106, which is subject to a lowerprocessing burden as a consequence of the decreased (or vanishing) framerate. While the system is in low-power mode, the control system 106monitors environmental illumination parameters like environmentalbrightness and/or reflectivity (step 204), either continuously or atcertain intervals, based on readings from the sensor(s) 118.

As long as the brightness and/or reflectivity (whichever is monitored)does not change significantly (e.g., remains below the specifiedthreshold), the system continues to be operated in low-power mode andthe brightness/reflectivity continues to be monitored. Once a change inbrightness and/or reflectivity is detected (step 206), the cameras (andassociated image-processing functionality of the control andimage-processing system 106) are switched into a high-frame-rate,high-power mode, in which motion of an object of interest 114 in theregion of interest 112 is continuously tracked (step 208). Frame ratesin this mode are typically at least 15 frames per second, and oftenseveral tens or hundreds of frames per second. Motion capture andtracking usually continues as long as the object of interest 114 remainswithin the region of interest 112. When the object 114 leaves the region112 (as determined, e.g., by the image-processing system 106 based onthe motion tracking in step 210), however, control system 206 switchesthe camera(s) back into low-power mode, and resumes monitoring theenvironment for changes in environmental illumination parameters likebrightness and/or reflectivity. The method 200 can be modified invarious ways. For example, in implementations where the cameras stillcapture images in the low-power mode, albeit at a low frame rate, anymotion detected in these images may be used, separately or inconjunction with changes in one or more environmental illuminationparameters such as environmental brightness or reflectivity, to triggerthe wake-up of the system.

FIG. 3 is a block diagram of an example computer system 300 foroperating a motion-capture system with reduced power consumption. Thecontrol and image-processing system 106 may generally be implemented insoftware, hardware, or a combination of both; FIG. 3 illustrates anexemplary computer implementation in block-diagram form. The computer300 includes a processor 302, a memory 304, a camera and sensorinterface 306, one or more mass storage devices 308, a display 309, userinput devices such as a keyboard 310 and a mouse 311, and a system bus(not shown) that facilitates communication between these components. Theprocessor 302 may be a general-purpose microprocessor, but depending onimplementation can alternatively be a microcontroller, peripheralintegrated circuit element, a CSIC (customer-specific integratedcircuit), an ASIC (application-specific integrated circuit), a logiccircuit, a digital signal processor, a programmable logic device such asan FPGA (field-programmable gate array), a PLD (programmable logicdevice), a PLA (programmable logic array), an RFID processor, smartchip, or any other device or arrangement of devices that is capable ofimplementing the steps of the methods of the technology disclosed. Themass storage devices 308 may be removable or non-removable, volatile ornon-volatile computer storage media. For example, a hard disk drive mayread or write to non-removable, non-volatile magnetic media. A magneticdisk drive may read from or write to a removable, non-volatile magneticdisk, and an optical disk drive may read from or write to a removable,non-volatile optical disk such as a CD-ROM or other optical media. Otherremovable/non-removable, volatile/non-volatile computer storage mediathat can be used in the exemplary operating environment include, but arenot limited to, magnetic tape cassettes, flash memory cards, digitalversatile disks, digital video tape, solid state RAM, solid state ROM,and the like. The storage media are typically connected to the systembus through a removable or non-removable memory interface.

The camera and sensor interface 306 can include hardware and/or softwarethat enables communication between computer system 300 and cameras suchas cameras 102, 104 shown in FIG. 1, as well as associated light sources(such as light sources 108, 110) and sensors (such as sensor(s) 118) ofFIG. 1. Thus, the camera interface 306 can, for example, include one ormore data ports 316, 318 to which cameras can be connected, as well ashardware and/or software signal processors to modify data signalsreceived from the cameras (e.g., to reduce noise or reformat data) priorto providing the signals as inputs to a motion-capture program 314executing on the processor 302. In some implementations, the camerainterface 206 can also transmit signals to the cameras, e.g., toactivate or deactivate the cameras, to control camera settings (framerate, image quality, sensitivity, etc.), or the like. Such signals canbe transmitted, e.g., in response to control signals from processor 302,which may in turn be generated in response to user input or otherdetected events. Further, the interface 306 can include controllers 317,319 to which light sources (e.g., light sources 108, 110) can beconnected. In some implementations, the controllers 317, 319 supplyoperating current to the light sources, e.g., in response toinstructions from the processor 302 executing the mocap program 314. Inother implementations, the light sources can draw operating current froman external power supply (not shown), and the controllers 317, 319 cangenerate control signals for the light sources, e.g., instructing thelight sources to be turned on or off or changing the brightness. In someimplementations, a single controller can be used to control multiplelight sources. Further, the interface 306 may include one or more sensordata ports 320 connected to and receiving data from the sensor(s) 118.

The memory 304 may be used to store instructions to be executed by theprocessor 302, as well as input and/or output data associated withexecution of the instructions. In particular, the memory 304 containsinstructions, conceptually illustrated as a group of modules, thatcontrol the operation of the processor 302 and its interaction with theother hardware components. An operating system directs the execution oflow-level, basic system functions such as memory allocation, filemanagement, and the operation of mass storage devices. The operatingsystem may be or include a variety of operating systems such asMicrosoft WINDOWS operating system, the Unix operating system, the Linuxoperating system, the Xenix operating system, the IBM AIX operatingsystem, the Hewlett Packard UX operating system, the Novell NETWAREoperating system, the Sun Microsystems SOLARIS operating system, theOS/2 operating system, the BeOS operating system, the MACINTOSHoperating system, the APACHE operating system, an OPENSTEP operatingsystem, iOS, Android or other mobile operating systems, or anotheroperating system of platform.

At a higher level, the memory stores software applications implementingmotion-capture and control in accordance herewith. These applicationsinclude, for example, the mocap program 314, which, when executed,performs motion-capture analysis on images supplied from camerasconnected to the camera interface 306. In one implementation, the mocapprogram 314 includes various modules, such as an object detection module322 and an object analysis module 324. The object detection module 322can analyze images (e.g., images captured via camera interface 306) todetect edges of an object therein and/or other information about theobject's location, and the object analysis module 324 can analyze theobject information provided by the object detection module 322 todetermine the 3D position and/or motion of the object. Examples ofimage-analysis methods for position determination and/or motion captureare described in U.S. patent application Ser. No. 13/414,485, filed onMar. 7, 2012, and U.S. Provisional Patent Application No. 61/724,091,filed on Nov. 8, 2012, which are hereby incorporated herein by referencein their entirety. The memory 304 may also store a camera-controlapplication 330, which receives, via the sensor data port(s) 320, ameasured brightness or reflectivity signal, compares this signal againsta specified threshold, and generates control signals for the cameras102, 104 and/or the light sources 108, 110 and sensor(s) 118 basedthereon. The control signals may, for example, serve to switch thecameras between low-power and high-power modes. Alternatively to beingimplemented in software, camera control may also be facilitated by aspecial-purpose hardware module integrated into computer system 300.

The display 309, keyboard 310, and mouse 311 can be used to facilitateuser interaction with computer system 300. These components can be ofgenerally conventional design or modified as desired to provide any typeof user interaction. In some implementations, results of motion captureusing the camera interface 306 and the mocap program 314 can beinterpreted as user input. For example, a user can perform hand gesturesthat are analyzed using mocap program 314, and the results of thisanalysis can be interpreted as an instruction to some other programexecuting on processor 300 (e.g., a web browser, word processor, orother application). Thus, by way of illustration, a user might useupward or downward swiping gestures to “scroll” a webpage currentlydisplayed on the display 308, to use rotating gestures to increase ordecrease the volume of audio output from a speakers, and so on.

It will be appreciated that computer system 300 is illustrative and thatvariations and modifications are possible. Computer systems can beimplemented in a variety of form factors, including server systems,desktop systems, laptop systems, tablets, smart phones or personaldigital assistants, and so on. A particular implementation may includeother functionality not described herein, e.g., wired and/or wirelessnetwork interfaces, media playing and/or recording capability, etc. Insome implementations, one or more cameras may be built into the computerrather than being supplied as separate components. Further, animage-processing functionality can be implemented using only a subset ofcomputer system components (e.g., as a processor executing program code,an ASIC, or a fixed-function digital signal processor, with suitable I/Ointerfaces to receive image data and output analysis results). Further,while computer system 300 is described herein with reference toparticular blocks, it is to be understood that the blocks are definedfor convenience of description and are not intended to imply aparticular physical arrangement of component parts. Further, the blocksneed not correspond to physically distinct components. To the extentthat physically distinct components are used, connections betweencomponents (e.g., for data communication) can be wired and/or wirelessas desired.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain implementations of the technologydisclosed, it will be apparent to those of ordinary skill in the artthat other implementations incorporating the disclosed implementationherein may be used without departing from the spirit and scope of thetechnology disclosed. For example, the systems and methods for reducingpower consumption in accordance herewith are not limited to motioncapture systems utilizing conventional edge detection orcontrast-enhanced edge detection as specifically described, but aregenerally applicable to any motion capture system, regardless of theimage-processing steps used to detect and track moving objects.Accordingly, the described implementations are to be considered in allrespects as only illustrative and not restrictive.

What is claimed is:
 1. A method of operating a motion-capture systemhaving at least one camera to minimize power consumption while assuringhigh-quality motion capture to track motions of an object, the methodincluding: monitoring using an optically sensitive sensor-a region ofinterest including a field of view of the at least one camera of amotion-capture system; wherein the region of interest is set as a volumeof approximately 1 meter in length and a light source positioned nearthe camera(s) of the motion-capture system to shine light onto theobject thereby enabling objects of interest to be distinguished frombackground objects using falloff of light intensity with distance; usingthe optically sensitive sensor, measuring brightness of light in a fieldof view of the at least one camera of the motion capture system at afirst measurement and a second measurement conducted subsequent to thefirst measurement; comparing brightness of light measured using thesensor at the second measurement and brightness of light measured usingthe sensor at the first measurement to determine a difference inbrightness of light measured at first and second measurements;determining that the difference in brightness of light measured usingthe sensor at first and second measurements exceeds a predeterminedthreshold indicating an object has entered the field of view of the atleast one camera of the motion-capture system between the firstmeasurement and the second measurement; responsive to determining thatthe difference in brightness of light measured using the sensor at firstand second measurements exceeds a predetermined threshold indicating anobject has entered the field of view, (i) detecting subsequent motionsof the object using the motion capture system; (ii) determining speed ofthe subsequent motions detected by the motion capture system, and (iii)switching the motion-capture system to operate in one of a set ofintermediate modes, each intermediate mode having a rate of imagecapture different from a rate of image capture of other intermediatemodes, the intermediate modes arranged between a low-power mode to ahigh-power mode; and repeating the (i) detecting, (ii) determining, and(iii) switching until the motion capture system determines that theobject has left the region of interest, to reconstruct 3D motion of theobject; thereby enabling the motion capture system to minimize powerconsumption while assuring high-quality motion capture to track motionsof the object.
 2. The method of claim 1, further including opticallysensitive sensor monitoring environmental reflectivity in the region ofinterest.
 3. The method of claim 2, wherein monitoring the environmentalreflectivity further includes blinking a light source and measuring abrightness of a reflection signal using the optically sensitive sensor.4. The method of claim 3, wherein the light source is turned off,dimmed, or blinked when the at least one camera operates in low-powermode.
 5. The method of claim 1, further including setting a threshold soas to be indicative of at least one of presence or movement of an objector person in the region of interest.
 6. The method of claim 1, furtherincluding setting a threshold so as to be indicative of geometricconfiguration of the region of interest.
 7. The method of claim 1,further including switching the motion capture system into the low-powermode responsive to receiving a user's command.
 8. The method of claim 1,further including successively decreasing processing of captured imagesin intermediate modes having successively reduced frame rates.
 9. Themethod of claim 1, further including monitoring brightness in the regionof interest at certain time intervals when in lowest power mode.
 10. Themethod of claim 1, further including tracking objects moving in theregion of interest using a frame rate of several tens or hundreds offrames per second at a high power mode.
 11. The method of claim 1,further including evaluating images captured by the at least one camerain low-power mode for presence or movement of an object of interestdependent on distinguishing object pixels from background pixels byapplying a brightness threshold to each image pixel.
 12. The method ofclaim 1, further including filtering out visible light so that onlyinfrared light is registered in images captured by cameras of the motioncapture system.
 13. The method of claim 1, wherein the low-power mode isa standby or sleep mode during which motion capture does not take place.14. A non-transitory computer readable medium storing instructions foroperating a motion-capture system having at least one camera to minimizepower consumption while assuring high-quality motion capture to trackmotions of an object, the instructions when executed by one or moreprocessors perform: monitoring using an optically sensitive sensor, aregion of interest including a field of view of the at least one cameraof a motion-capture system; wherein the region of interest is set as avolume of approximately 1 meter in length and a light source positionednear the camera(s) of the motion-capture system to shine light onto theobject thereby enabling objects of interest to be distinguished frombackground objects using falloff of light intensity with distance; usingthe optically sensitive sensor, measuring brightness of light in a fieldof view of the at least one camera of the motion capture system at afirst measurement and a second measurement conducted subsequent to thefirst measurement; comparing brightness of light measured using thesensor at the second measurement and brightness of light measured usingthe sensor at the first measurement to determine a difference inbrightness of light measured at first and second measurements;determining that the difference in brightness of light measured usingthe sensor at first and second measurements exceeds a predeterminedthreshold indicating an object has entered the field of view of the atleast one camera of the motion-capture system between the firstmeasurement and the second measurement; responsive to determining thatthe difference in brightness of light measured using the sensor at firstand second measurements exceeds a predetermined threshold indicating anobject has entered the field of view, (i) detecting subsequent motionsof the object using the motion capture system; (ii) determining speed ofthe subsequent motions detected by the motion capture system, and (iii)switching the motion-capture system to operate in one of a set ofintermediate modes, each intermediate mode having a rate of imagecapture different from a rate of image capture of other intermediatemodes, the intermediate modes arranged between a low-power mode to ahigh-power mode; and repeating the (i) detecting, (ii) determining, and(iii) switching until the motion capture system determines that theobject has left the region of interest, to reconstruct 3D motion of theobject; thereby enabling the motion capture system to minimize powerconsumption while assuring high-quality motion capture to track motionsof the object.
 15. A motion-capture system, including: at least onecamera for capturing images of an object within a field of view thereof;an image-analysis module comprising a processor coupled to a memorystoring instructions that, when executed by the processor, track motionof the object based on the images and interprets as input a user's handgestures tracked; an optically sensitive sensor for measuring brightnessof light within the field of view of the at least one camera at a firstmeasurement and a second measurement subsequent to the firstmeasurement; and a control module comprising a processor coupled to amemory storing instructions that, when executed by the processor,perform: monitoring using the optically sensitive sensor, a region ofinterest including a field of view of the at least one camera of amotion-capture system; wherein the region of interest is set as a volumeof approximately 1 meter in length and a light source positioned nearthe camera(s) of the motion-capture system to shine light onto theobject thereby enabling objects of interest to be distinguished frombackground objects using falloff of light intensity with distance; usingthe optically sensitive sensor, measuring brightness of light in a fieldof view of the at least one camera of the motion capture system at afirst measurement and a second measurement conducted subsequent to thefirst measurement; comparing brightness of light measured using thesensor at the second measurement and brightness of light measured usingthe sensor at the first measurement to determine a difference inbrightness of light measured at first and second measurements;determining that the difference in brightness of light measured usingthe sensor at first and second measurements exceeds a predeterminedthreshold indicating an object has entered the field of view of the atleast one camera of the motion-capture system between the firstmeasurement and the second measurement; responsive to determining thatthe difference in brightness of light measured using the sensor at firstand second measurements exceeds a predetermined threshold indicating anobject has entered the field of view, (i) detecting subsequent motionsof the object using the motion capture system; (ii) determining speed ofthe subsequent motions detected by the motion capture system, and (iii)switching the motion-capture system to operate in one of a set ofintermediate modes, each intermediate mode having a rate of imagecapture different from a rate of image capture of other intermediatemodes, the intermediate modes arranged between a low-power mode to ahigh-power mode; and repeating the (i) detecting, (ii) determining, and(iii) switching until the motion capture system determines that theobject has left the region of interest, to reconstruct 3D motion of theobject; thereby enabling the motion capture system to minimize powerconsumption while assuring high-quality motion capture to track motionsof the object.